A turbo-supercharger driven by a turbine provided in the path of engine exhaust is known to improving the output of an engine.
The efficiency of the compressor of such a turbo-supercharger can be, as is commonly known, indicated by parameters comprising the pressure ratio of the top and bottom flow of the compressor, and the gas flow rate that passes the compressor. When the pressure ratio deviates from the appropriate pressure ratio range (normal operation range) that corresponds to the gas flow rate, and enters an abnormal operation range, undesirable phenomena, such as surging, choking or over-speeding of the turbine, are experienced, whereby the operating condition of the compressor should be established so as not to enter the abnormal operation range.
For example, when sudden deceleration is effected by releasing the accelerator pedal during high-speed steady driving, the fuel injection rate is decreased, thereby causing a rapid reduction in the exhaust flow rate. During this time, the operating condition of the compressor sometimes may enter the abnormal operation range, thereby generating surging.
In addition, an EGR (Exhaust Gas Recirculation) device for introducing only part of the exhaust into the inlet path according to the driving conditions, is known, in which a pipe connecting between the exhaust path and an inlet path (EGR pipe) and an EGR valve that controls the flow rate of the EGR pipe are provided.
In such an EGR device, the exhaust is introduced to the inlet path due to the differential pressure between the exhaust path and the inlet path, and then the aperture (open degree) of the EGR valve is adjusted so that the amount of EGR increases when the driving conditions shift to the low load side. However, when a vehicle speed is reduced in the normal driving condition so that the fuel is reduced by releasing the accelerator pedal, the exhaust pressure decreases, and therefore, the EGR gas may backflow via the EGR pipe into the turbine so that it may cause a surge noise.